I located the tile that is causing the problem, bc after removing it, construct doesn't complain anymore.
So I looked at the tiles collision mask... there's only three points in it, a small triangle.
I've placed the triangle in the middle of the tile instead of at the edge. Didn't solve the problem.
I've also placed any collision mask points close to the edge of neighboring tiles a bit further away. Didn't solve anything.
As I've pointed out, if I delete it the problem goes away, but then I loose the artwork for that specific tile as well, and the tile is part of a "spliced large foreground tile".
The message Construct gives me when I run the layout begins with the following line: "assertion failure: physics polygon separation resulted in no polys".
Grateful for any input, thx!